1. Field of the Invention
The present invention relates to a method of producing xylylenediamine which is useful as a resin curing agent and a raw material thereof, a polyamide resin, an isocyanate intermediate raw material, or the like.
2. Description of the Related Art
In recent years, high purity xylylenediamine having low cyanobenzylamine content is required as an isocyanate intermediate raw material, in particular. In such an application, high purity xylylenediamine having a cyanobenzylamine content of 0.02 wt % or less is required. Thus, a method of producing high purity xylylenediamine having low cyanobenzylamine content industrially and advantageously is required.
Xylylenediamine is known to be obtained through catalytic hydrogenation of phthalonitriles in the presence of liquid ammonia and/or an organic solvent (see JP 51-24494 A and JP 2002-105035 A). Phthalonitriles are compounds having relatively high melting points, and thus use of liquid ammonia or an organic solvent is required. In particular, liquid ammonia has conventionally been known to have an action of suppressing an undesirable side reaction in which an imine intermediate formed in a hydrogenation reaction and a nitrile group reacts to form an oligomer or the like. Thus, a large amount of liquid ammonia is often used.
Further, the catalytic hydrogenation reaction is a successive reaction in which phthalonitriles convert into cyanobenzylamine and then cyanobenzylamine converts into xylylenediamine. Thus, a trace amount of cyanobenzylamine always remains in a reaction system.
In the case where the catalytic hydrogenation reaction is conducted in a one-pass flow reaction or a batch reaction, a large amount of liquid ammonia must be used for suppressing a side reaction as described above. Thus, an apparent reaction rate is reduced due to reduction in a substrate concentration, and cyanobenzylamine is liable to remain in the reaction mixture. WO 2005/026098 describes a circular reaction system in which a part of reaction mixture from an outlet of reactor is mixed with a freshly fed raw material and returned to the inlet of the reactor for suppressing a use amount of a solvent. However, the substrate concentration is reduced even in this case, so cyanobenzylamine is liable to remain.
Cyanobenzylamine is an unstable substance which is liable to be colored and decomposed when left at stand, and forms a viscous substances through polymerization. Cyanobenzylamine in a state included in xylylenediamine also exhibits similar behaviors above mentioned and generally has a small difference in boiling points with that of corresponding xylylenediamine. Thus, separation of cyanobenzylamine through normal distillation involves difficulties.
Examples of a known method of producing xylylenediamine having low cyanobenzylamine content include: a method involving subjecting phthalonitriles to catalytic hydrogenation in the presence of liquid ammonia and/or an organic solvent, removing a solvent, and subjecting the thus-obtained crude xylylenediamine (a xylylenediamine having high cyanobenzylamine content) to alkali treatment (JP 45-14777 A, for example); and a method involving bringing crude xylylenediamine into contact with a catalyst containing an iron oxide or an oxide of iron and chromium in the presence of water (JP 57-27098 A, for example).
However, in the method involving alkali treatment, xylylenediamine having low cyanobenzylamine content can be obtained, but wastewater containing alkali is discharged. This wastewater is neutralized and then subjected to activated sludge treatment or incinerated treatment. However, there are many problems in such a treatment because of the alkali contained in the wastewater. The method involving bringing crude xylylenediamine into contact with a catalyst containing an iron oxide or an oxide of iron and chromium in the presence of water is industrially disadvantageous because the method requires a separate step of distilling off water used in a reaction or the like. Both methods are inefficient because cyanobenzylamine as an intermediate is converted into a high boiling point substance.
JP 2004-292435 A describes high purity xylylenediamine having low cyanobenzylamine content can be produced at high efficiency through hydrogenation of phthalonitriles in the presence of a solvent through two steps including: a step (a) of conducting a hydrogenation reaction until a conversion of nitrile groups reaches within a range of 90 mol % or more and less than 99.9 mol %; and a step (b) of conducting a hydrogenation reaction at a reaction temperature which is 10° C. or more higher than that of the step (a) to a higher nitrile conversion than that of the step (a) and 99.5 mol % or more.
This method is one method of obtaining high purity xylylenediamine having low cyanobenzylamine content. However, an undesirable side reaction takes place due to a high reaction temperature, and thus the nitrile conversion and reaction temperature in the step (a) must be adjusted for suppressing the side reaction, thereby involving complications in operation. Further, as a hydrogenation reaction is always conducted in the presence of a reaction solvent, and thus a considerable amount of a catalyst is required due to a low substrate concentration.